It depends alot on the medium your growin in, and what kind of chems your talkin about. If its just a normal fert, about 3 weeks is the norm, depending on the growth progress till then. And start with a diluted mixture, like 1/4, so you dont burn out your babies. Like I said, man, read as much as you can. Type your question into a search engine, and go to multiple sites to get answers, andgo from there. Pushit has givin me good advice. This site seems to be the best. Very smart growers. Nice people willing to help. Good luck man. Cant wait to see pics if you decide to post em. I got a log on here too, if you wanna check it out from an outdoor grow perspective. Happy growin..... Its addictive.....